The present proposal seeks support for the Second International Workshop on Human Leucocyte Differentiation Antigens to be held in Boston September 17-20, 1984. The initial meeting was convened in Paris November 1982 under the auspices of an international organizing committee with support from IUIS, WHO and INSERM and has a three-fold purpose. First, it serves to encourage a free world-wide exchange of monoclonal antibody reagents to human leucocyte differentiation antigens, thus permitting the orderly characterization of antibody reactivities and the surface molecules which they identify. Data is catalogued and published in handbook form for future reference, analogous to the HLA workshops. Secondly, the meeting serves as a vehicle for in depth analysis of T lymphocytes, B lymphocytes, stem cells and myeloid-macrophages and leukemia exploiting monoclonal antibody technology. Within the T cell area, for example, specific sessions will be devoted to recognition structures, regulatory elements, activation antigens, polymorphic surface determinants and phylogenetic studies. Technologies to be employed in addition to monoclonal antibodies will include T cell clones, functional analyses, and molecular biology. Antibodies entered into this study by individual laboratories will be available for each of these workshop efforts. This format will allow for exchange of information among investigators and the opportunity to test reagnets created by the various participants. It is hoped that new insights into existing materials will be generated, fruitful collaborations developed, and reduplication of research efforts prevented. Given the central role of cell surface structures in cell biology and clinical medicine, such a forum will be important for immunologists, cell biologists, molecular biologists, biochemists, rheumatologists, endocrinologists, oncologists, hematologists, neurologists, allergists, transplantation biologists and others. Third, the workshop will help in the establishment of a uniform nomenclature based on the results of investigators' findings. Proposals will be presented to a WHO nomenclature committee prior to adoption of formal designations.